Fluidandelectrolytedisordersareobservedcommonlyinclinicalpractice(e.g.,inpatientswithvomitingordiarrhea,orboth).Inmostinstances,thesedisordersareself-limited,andcorrectionofthedisorderoccurswithoutneedforintervention.However,moresevereorprolongeddisordersmaynecessitatefluidreplacementtherapy.Suchtherapymaybeadministeredorally,withspecialelectrolytesolutions,orintravenously,withfluid.
Intravenoussolutionsareavailableinmanyformulations.Thetypeoffluidadministeredtoaparticularpatientisdictatedbythepatient’sneed.Forexample,ifanincreaseinthepatient’svascularvolumeisnecessary,asolutioncontainingsubstancesthatdonotreadilycrossthecapillarywallisinfused(e.g.,5%proteinordextransolutions).Theoncoticpressuregeneratedbythealbuminmoleculescausesfluidtoberetainedinthevascularcompartment,whichexpandsitsvolume.ExpansionoftheECFisaccomplishedmostoftenwithisotonicsalinesolutions(e.g.,0.9%NaClorlactatedRingersolution).Asalreadynoted,administrationofanisotonicNaClsolutiondoesnotresultinthedevelopmentofanosmoticpressuregradientacrosstheplasmamembraneofcells.Therefore,theentirevolumeoftheinfusedsolutionremainsintheECF.
Patientswhosebodyfluidsarehyperosmoticneedhypotonicsolutions.ThesesolutionsmaybehypotonicNaCl(e.g.,0.45%NaCl)or5%dextroseinwater(D5W).AdministrationoftheD5WsolutionisequivalenttotheinfusionofdistilledwaterbecausethedextroseismetabolizedtoCO2andwater.AdministrationofthesefluidsincreasesthevolumesofboththeICFandECF.Inaddition,patientswhosebodyfluidsarehypotonicneedhypertonicsolutions.ThesearetypicallyNaCl-containingsolutions(e.g.,3%or5%NaCl).ThesesolutionsexpandthevolumeoftheECFbutdecreasethevolumeoftheICF.Otherconstituents,suchaselectrolytes(e.g.,K+)ordrugs,canbeaddedtointravenoussolutionstotailorthetherapytothepatient’sfluid,electrolyte,andmetabolicneeds.
distribution of cations and anions between these two compartments by the Gibbs-Donnan effect (see the section “ ” for details), this effect is small, and the ionic compositions of the interstitial fluid and plasma can be considered to be identical.
Maintenance of Cellular Homeostasis
Normal cellular function requires that the ionic composition of the ICF be tightly controlled. For example, the activity of some enzymes is pH dependent; therefore, intracellular pH must be regulated. In addition, the intracellular composition of other electrolytes is similarly held within a narrow range. This is necessary for the establishment of the membrane potential, a cell property especially important for the normal function of excitable cells (e.g., neurons and muscle cells) and for intracellular signaling (e.g., intracellular [Ca++]; see for details). Finally, the volume of cells must be maintained because shrinking or swelling of cells can lead to cell damage or death. The regulation of intracellular
K+ 3.5-5.0mEq/L120-150mEq/L
Ca++ 2.1-2.8(total)mmol/L 1.1-1.4(ionized)≈10−7M(ionized)